Anal. Chem. 1985, 57, 1457-1461
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Simultaneous Flame Photometric Determination of Lithium, Sodium, Potassium, and Calcium by Flow Injection Analysis with Gradient Scanning Standard Addition Fang Zhaolun,' Joel M. Harris,*2Jaromir Ruzicka, and Elo H. Hansen Chemistry Department A, The Technical University of Denmark, Building 207, 2800 Lyngby, Denmark
The gradient scanning flow Injection technique has been appiled to the simultaneous flame photometrlc determination of LI, Na, K, and Ca in soil extracts and tap water. A fast scanning monochromator and a storage oscilloscope have been employed to obtain spectra in the range 350-800 nm on dtfferent sections of the Injected sample zone to optlmlze intensity ranges for ail analytes. Relative standard deviations were In the range 2.5-4.6 %. A novel Standardization method has been proposed to achieve multicomponent standard addition at different sampiehtandard ratlos in a single injection.
Gradient techniques of flow injection analysis have provided new possibilities in automated solution handling by extensively exploring the wealth of information contained within the controllably dispersed sample zone (1). Among those approaches which proved to be successful in various applications, the gradient scanning method, by which physical parameters such as absorbance vs. wavelength or current vs. potential are repeatedly and rapidly scanned along a dispersed sample zone, has an excellent potential for multicomponent analysis ( 2 ) . The advantages of gradient scanning are further demonstrated in the present work by multicomponent analysis performed by emission spectrometry using a fast scanning monochromator. Besides providing the possibility of simultaneous multicomponent analysis (3)and internal standardization in a single sample injection and single scan, fast scans can be repeatedly made on different sections of the sample zone to provide optimum concentration ranges for the detector. Furthermore, a multielement standard addition procedure has been developed based on merging standard solutions with the sample zone and by scanning the resulting concentration profiles. In the present work, a fast scanning monochromator coupled with a digital oscilloscope was used to scan analyte emission spectra from an air-acetylene flame. Sodium, potassium, and calcium were determined simultaneously in soil extracts and in tap water over wide concentrationranges using lithium as internal reference by scanning on two different levels on the dispersed sample zone for each injection. The relative standard deviations were in the range 2.5-4.6%. A novel standard addition method has been proposed which utilizes the information of the sample gradient and the fast scan capability of the system and permits the execution of multicomponent standard addition at different sample/ standard ratios from a single injection.
EXPERIMENTAL SECTION Apparatus. The monochromator was a Rofin Model 6000 with an Ebert mounting have a 0.1-m focal length and 1200 Present address: Institute of Forestry & Soil Science, Academia Sinica, P.O. Box 417, Shenvana. China. Present address: DeparimGt of Chemistry, University of Utah, Salt Lake City, UT 84112.
line/mm grating blazed at 300 nm, capable of scanning from 350 nm to 800 nm in 5 ms every 100 ms. The matched entrance and exit slits determined the spectral bandwidth of 10 nm. The image of the air-acetylene flame from an atomic absorption slot type premix burner arranged lengthwise was focused on the entrance slit of the monochromator with an f = 200 mm glass lens. The detector was a silicon photodiode amplified by a current to voltage converter having a lo8 s2 feedback resistance. The output of the amplifier was connected to a Tektronix 5111A digitizing storage oscilloscope, with sensitivity set at 200 mvldivision. After a predetermined delay following sample injection, digitizing by the oscilloscope was initiated by a timer triggered by a microswitch on the sampling valve. Following storage of the initial spectrum in the scope memory, a second spectrum could be digitized at a predetermined delay controlled by the viewing time of the oscilloscope. The two spectra, thus obtained on different sections of the FIA peak, were plotted on an X-Y recorder, Bruel and Kjaer Type 2308. An atomic absorption spectrophotometer, Varian AA-1275, was operated in the emission mode in order to compare the multiwavelengthmeasurement with conventional flame emission under the same flame conditions and to record the FIA response at fixed wavelengths as in Figure 5. A peristaltic pump, Ismatec Model Mini-S-840, with Tygon pump tubes and a PVC-Teflon two-layer, minirotary injection valve, locally constructed, were used in the flow system. The sample volume was 40 pL for normal operation and 100 ILL for the standard addition method. All connecting lines and the sample loop were made from 0.5 mm i.d. microline tubing (Thermoplastics Scientific). The line length from the valve to the nebulizer was 10 cm. The arrangements of the system for normal and standard addition operations are shown in Figure 1, parts a and b, respectively. Reagents. All chemicals were analytical reagent grade. Distilled water was used in the preparation of all solutions. Standard stock solutions of Ca, Na, and K were prepared at a concentration of 1 g L-l and diluted to 80, 60,40, and 20 mg L-l standard solutions with water or 1 M ammonium acetate (pH 7). The latter was used for the analysis of sol extracts without standard addition. Whenever internal standardization was performed, 20 mg L-' Li was added to each standard and sample as internal reference. The carrier stream generally was distilled water. For the standard addition procedure, an aqueous solution containing Ca, Na, and K each at a level of 80 mg L-l was used as carrier. In this case, Li was added to the samples, to provide a check on the dispersion of the peak sections being measured. The soil extractant was 1 M ammonium acetate (pH 7). Procedure. Tap water samples were injected directly into the system without further treatment. The 1 M ammonium acetate soil extracts were prepared by a standard procedure ( 4 ) ,where 2 g of soil was extracted and leached to obtain a 100-mL extract. Lithium was added as an internal reference to each extract before diluting to the final volume, such that the final solution contained 20 mg L-* of Li.
0003-2700/85/0357-1457$0 1.50/0 0 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL.
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NO. 7, JUNE
1985
(a) C
(b)
I
400
500
600
nm
700
800
Figure 2. Three-dimensionalgradient scanning recording of a 40-fiL tap water sample. Time between successive scannings was 0.3 s. Wavelengths of peaks, from lefl to right, are as follows: CaOH. 554 nm; Na, 589 nm: CaOH. 622 nm: K, 767 nm.
SC
W
FIgure 1. Flow system and setup for simultaneous muiticomponent gradient scanning by flame photometric F I A (a) calibration curve method and (b) standard addition method. S, sample; C. carrier scW n ; SC,standard d h n c a m W. waste: L. sample loop: V. M: P, pump: F, flame nebulizer-burner, T, timer: M. scanning moncchromator: 0, storage oscilloscope: R. X-Y recorder. Note that the sample loop moves upon injection. The spectra of the predetermined section or sections of the FIA peak were scanned, stored, and recorded. The peak heights of Ca, 622 nm (CaOH hand peak), Na, 589 nm, K, 767 nm, and Li, 671 nm, were read and calibration curves were constructed hy relating concentration to intensity or log concentration to log intensity. The latter relationship was used to obtain a linear calibration plot when nonlinearities arise for Na and K due to self-absorption. In the standard additon mode, an 80 wg mL-' Ca, Na, and K standard solution without Li was used as the carrier. The flow system for the valve was modified as in Figure l b so that distilled water was aspirated into the flame prior to the injection. Upon injection, the standard solution carrier first propels the sample into the flame, following which a steadystate standard response is produced. Intensities of sample and sample-plus-standard were recorded a t identical dispersion values, D,located a t the front and tail sections of the sample zone. Following injection, the valve is returned to the filling position which starts the flow of distilled water into the flame which removes the standard solution carrier within 5 s in preparation for another sample injection. RESULTS AND DISCUSSION Multicomponent Analysis by the Scanning FIA System. The three-dimensional presentation of the emission spectrum of a tap water sample along the gradient of ita FIA peak in Figure 2 shows the capability of the gradient scanning technique in atomic emission spectrometry. The scans from 40-pL volume injections were collected a t 300-ms intervals after injection. Since the oscilloscope could store a maximum of two scans per injection, the figure was constructed from a series of injections scanned a t different delay times. Although the spectral scans in Figure 2 are from different injections, the continuity of the resulting peaks is evidence of the reprcducihility of the method. The results shown in Figure 2 also indicate that multicomponent analysis, including internal standardization, could be performed a t optimum signal intensity for each element by selecting appropriate sections of the dispersed zone. Figure 3 shows two scans of a 20 mg L-' standard, one taken at the FIA peak maximum (1.2 s after injection), the other on the tailing part of the peak (2.5 s after
L 400 l
1
500
600
700
800
nm Flgure 3. FIA gradient scanning of 40 fiL of 20 mg L-' standard solutim for Na. K. Ca. and Li at two dispersion levels scanned at 1.25 sand 2.5 s after injection. The K 767 nm emission was off scale for the first scan. The calibration curves for Na 589 nm at the two dispersions are shown in the insert. Anhough the ratio of D , to D , is 5. the ratio of the peak heights is much less due to self-absorption of higher concentrations. injection). While the sodium and calcium peaks are in favorable ranges in the first scan, the potassium response was off-scale and would he impossible to analyze in the same injection had not the second scan been taken. Because of the limited data storage capability of the oscilloacope and rather slow recording speed of the X-Yrecorder, the time between injections, -2 min, was much longer than that achievable hy the scanner, 0.1 s, or required by the FIA system, -5 8. However, the purpose of the present work is to show the feasibility of such an approach, understanding that sampling frequency could he much improved by storing and processing the data with a computer. Precision of the Gradient Scanning Technique. The gradient scanning technique involves the measurement of one physical parameter (intensity) against two other rapidly changing parameters (wavelength, analyte concentration). This approach ohviously requires an accuracy in timing and reprcducihility of zone dispersion combined with low electronic and flame noise levels. A question would arise as to the precision which could be attained by means of such a technique. Therefore, the precision of the method was evaluated hy 11 scans on repeated 40-pL injections of a soil extract recorded at peak maximum (Figure 4). Despite the short term noise in the flame source and the high scanning speed of the monochromator which allows little averaging of the signal, the precision obtained was quite satisfactory for soil analysis. The relative standard deviations of peak height for Na, Ca, and K without an internal reference were 3.9,4.6, and 3.2%, re-
ANALYTICAL CHEMISTRY, VOL. 57. NO.7. JUNE 1985
400
500
600
700
8C
nm mure 4. Replicate scans recOrded on the peak maximum of a soil
extract sample solution which was successively injected 11 times. spectively. By employing Li as an internal reference, the precision for Na improved to 2.5% hut no improvement could he obtained for the other analytes. The contribution to the peak height uncertainty from the asynchronous scanning of the monochromator with respect to the injection event was estimated by modeling the FIA peak as a Gaussian function of 1.4 s, full width a t half maximum. Since the interval between successive scans of the spectrum is 100 ms, the average timing error, *50 ms, would produce a drop in intensity of
1459
only 0.4% relative to the peak value. Therefore, scan timing is contributing little to the observed uncertainty of the above results. Gradient Scanning Standard Addition. A number of standard addition techniques for FIA, based on different principles, have been developed or proposed. These include the generalized standard addition approach for emission spectrometry (5), the injection of standard solutions into a sample carrier in AAS (6)and ICPES (3, the merging of an injected sample zone and a standard solution zone (69,and the incorporation of zone sampling (9). In the present work, a new standard addition method, which allows simplicity in sample manipulation and the design of the flow system and which fully utilizes the gradient scanning capability of the detection system, is proposed. The flow system is illustrated in Figure l h and can easily he adapted from the svstem in Figure l a without use of additional components. Standard solutions without an internal reference were used as carrier solution which was directed to waste during the sampling stage, while distilled water was aspirated into the burner. When the valve was turned, the sample was propelled hy the standard carrier stream with water preceding the sample zone, the standard solution being gradually dispersed into the sample zone (Figure 5a, top). Following the dispersion, pairs of those gradient sections of the sample zone having identical dispersions, D, and hence the same matrix composition and analyte concentrations, were selected on the rising and falling portions of the FIA peak a t delay times, t , and t,, from the time of injection. The emission intensity of the Li, added as an internal reference to the samples, served as a check on the equal sample dispersion condition a t two gradient sections scanned. The Li signal heights must he identical a t t , and t,. Three pairs of these gradient sections, a t different dispersion levels, D,, D2, and D,, are shown in Figure 5a. If the sample zone is large enough to prevent the excessive penetration of the standard carrier solution, the
x c ._ VI
> + ._
a,
a: +
VI
C
C
-
1
C -
C
I
tl
t2
Scan
-
5bO
6bO
7bO
nm
dl0
I
I
Flpure 5. Gradient scanning standard addition. (a)Time-intensity recordings at 622 nm of an 80 mg L-' standard Ca soiution injected as a 100-pL sampie wHh water as carrier to show the dispersion of the sample zone (A). The broken lines indicate the times and dispersions when idantical D values can be obtained on the rising and failing pa* of the FIA cuwe. At the top is a schematic diagram of me conditions in the flow channel immediately prior to nebulization. B is a recording of the same standard used as carrier and distilled water injected as sample. Note that the standard solution is not Usperssd into the risiq prt of the sample zone. W, water; S, sample; SC; standard calrier. t,. 1,. D,. D,, and D , scanning times and dispersions for points of identical D values. (b) Time-intensity recordings at 622 nm of 100-pL soil extract samples with 80 mg L-' Ca as carrier. Cuwes 1, 2. and 3 are soils wiih high, medium. and low calcium concentrations. Times 1 , and t, fw C U N ~2 are the points where
scans were taken at identical dispersions for the sample zone. H. and H , are the responses obtained. (c)Waveienflh-intensty scans at f and
t , of sample 2 in (b). Note the identical heights of the Li 671 nm internal reference line which acts as a check on the equal sample dispersions of the two scanning points.
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
Table I. Results for the Analysis of Soil Ammonium Acetate Extractsn
53 17 29 31 30 22
2.0 2.5 3.5 5.8 4.4 3.5
2.2 2.5 3.5 5.6 4.4 3.5
4.0 2.3
4.5 2.0
1.1
1.1 1.1 1.0
1.3 1.1 1.1
1.1
114 16.5 47 15.0 12.0 15.0
110
17.5 47 14.5 13.0 15.0
85 18.4 48 15.2 13.7 15.8
Results expressed are the concentrations in the final extract. *Conventionalflame photometric method. gradient section on the rising part of the peak diluted with distilled water is free of standard solution, while the tailing section is diluted entirely with standard. This was accomplished by injecting a somewhat larger sample volume of 100 hL. Thus, if the dispersion of the corresponding sections of the sample zone having identical D values were known, the amount of standard added to the sample section a t t2can be deduced
l/Dst = 1 - l/D,,
(1)
where D,, is the dispersion of the sample at times tl and t 2 and D,, is the dispersion of the standard at time t 2 on the tailing part of the sample peak. The dispersion D of the selected gradient section was determined by injecting a standard solution into a water carrier and measuring the response H1 at the preselected delay time for the particular section and relating it to the response, H,, of the same solution aspirated into the nebulizer at the same carrier flow. The sample dispersion, D,,, is, therefore
When a standard addition was performed by using a standard solution as the carrier stream, the responses at two identical dispersion points of the sample zone, tl and t2,were measured, giving H, and Hf for the responses on the rising and falling parts of the peak, respectively; see curve 2 in Figure 5b. Assuming a linear calibration curve, the heights, HI and Hf can be related to the concentration of the analyte in the sample, C,,, and standard solution, Dst, by the following equations:
HI = kC,,d
Hf= k[C,,d
+ Cst(l - d ) ]
(3)
(4)
where k is a constant and the dilution factor, d, is related to the sample dispersion by d = l/Dgm.Thus
(5)
H,C,,d
+ HIC,t(l - d ) = HfC,,d
Since the concentration of the standard solution is known and the dispersion of the section chosen is measured according to eq 2, the expression CSt(l- d ) / d is a known constant, K. Hence
indicates that the sample concentration can be determined
by standard addition upon a single injection by measuring the response, Hf and HI, a t times tl and t 2 , respectively. As different sections on the gradient will provide different sample/standard ratios, the ratio could be optimized for precision by selecting the most suitable section. While consistency in the results would be expected irrespective of the section selected, larger errors might be expected when the sample/ standard ratio is much greater or much less than unity. A soil extract was analyzed for calcium by the standard addition procedure using 80 mg L-l Ca standard solution as the carrier, and the results were obtained at three different dispersion levels, Spectra were scanned at tl-t2 values of 0.8-3.8,l.O-3.2, 1.2-2.9 s, corresponding to sample dispersions, D,,, of 5, 2.1 and 1.6, respectively. The results obtained, 48,45, and 47 mg L-' Ca, were consistent with a linear response over the large range in sample/standard ratios. To illustrate the method further, the standard addition FIA peak was recorded at a fixed wavelength for Ca (622 nm). In Figure 5b, the recordings of three soils extracts having low, medium, and high calcium concentrations are shown. The 80 mg L-l Ca standard was used as carrier. The gradient scanning recordings obtained a t tl and t 2 for sample 2 are shown in Figure 5c. Although Na and K were also included in the standard addition for the purpose of illustration, the technique could not be applied to these elements because of nonlinearity of the calibration curves. Simultaneous Analysis of Potassium, Sodium, and Calcium in Ammonium Acetate Extract of Soils. Ammonium acetate extracts of soils were analyzed for potassium, sodium, and calcium in a single injection by the gradient scanning technique and for calcium by the standard addition procedure. The results were compared to those obtained with a conventional flame emission procedure and are shown in Table I. The results are generally in good agreement. The calcium content obtained by standard addition for one of the samples (no. 53) was considerably lower than the result obtained directly with a calibration curve. The sample was analyzed by using conventional standard addition giving a result of 78 mg L-l which was in better agreement with the gradient scanning standard addition method. This implies a positive interference from the sample matrix (probably Mg) which is not corrected when using a calibration curve prepared from standards alone. CONCLUSIONS Flow injection technique serves in the present work only as a means of transport for the sample material into the flame, and therefore, one might expect that it would merely allow an increase of sampling frequency. With the aid of a rapid scanning monochromator, however, additional data have been obtained along the wavelength axis, yielding a dimension which serves as a basis for multielement determination. Furthermore, the high reproducibility of FIA and the flexibility of the gradient approach allowed addition of another dimension to the matrix of collected data. By means of the zone penetration technique, sample and standard solutions
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Anal. Chem. 1985, 57, 1461-1464
were dispersed within each other with sufficient reproducibility that standard addition could be performed over a wide, controllable range of standard/analyte concentration ratios. I t is interesting to observe that the new variant of the gradient technique is partially based on the underlying concept of FIA titration ( I ) , where elements of fluid with identical D values are exploited, and partially on the concept of zone penetration ( 5 ) as designed for selectivity measurements, where elements of fluid with identical delay times t were used. Thus, another variation of the concept dispersed sample zone has been discovered and exploited. Its use is not limited to flame methods, as reaction based assays, like spectrophotometry, would equally benefit from the novel way of performing standard addition. Instruments equipped with photodiode arrays and appropriate fast data collection and storage facilities should increasingly find use in future applications. Registry No. Li, 7439-93-2; Na, 7440-23-5;K, 7440-09-7; Ca, 7440-10-2;water, 7732-18-5.
LITERATURE CITED Ruzicka, J.; Hansen, E. H. Anal. Chim Acta 1983, 145, 1. Janata, J.; Ruzicka, J. Anal. Chim. Acta 1982, 139, 105. Greenfield, S. Spectrochim. Acta, Part 8 1983, 388, 93. Black, C. A. "Methods of Soil Analysis, Part 2"; American Society of Agronomy, Inc.: Madison, W I , 1965; pp 894-895. (5) Zagatto, E. A. G.; Jacintho, A. 0.; Krug, F. J.; Reis, B. F.; Bruns, R . E.; Arujo, M. C. U. Anal. Chim. Acta 1983, 145, 169. (6) Tyson, J. F.; Appleton, J. M.; Idris, A. B. Anal. Cbim. Acta 1983. 745, 159. (7) Israel, Y.; Barnes, R. M. Anal. Chem. 1984, 56, 1188. (8) Hansen, E. H.; Ruzicka, J. Anal. Chim. Acta 1983, 148, 111. (9) Gine, M. F.; Reis, B. F.; Zagatto, E. A. G.; Krug, F. J.; Jacintho, A. 0. Anal. Chim. Acta 1983, 155, 131. (IO) Ramsing, A. U.; Ruzicka, J.; Hansen, E. H. Anal. Chim. Acta 1981, 129, 1.
(1) (2) (3) (4)
RECEIVED for review August 10, 1984. Resubmitted March 11,1985. Accepted March 11,1985. The authors express their gratitude to the Danish National Council for Scientific and Industrial Research for financial assistance to Z. Fang and J. Harris and to the Academia Sinica for grant of leave to Z. Fang.
Dicyclohexylcarbodiimide as a Cleaving Agent for Colorimetric Determination of Pyridyl and Pyrimidinyl Compounds Sheng-Chih Chen School of Pharmacy, China Medical College, 91 Hsueh Shih Road, Taichung, Taiwan, Republic of China
By use of dlcyclohexylcarbodAmIde (DCC) and dimethylbarblturic acid (DMBA) as reagents, a colorlmetric method for the deterrninatlon of pyrldyl and pyrlmldlnyl compounds has been established. DCC breaks the pyrldlne or pyrlmldine ring to afford giutaconaldehyde or maionaldehyde and then reacts wlth DMBA to produce chromophores. These heterocycles could be determined by measuring the chromophores. The reiatlve standard devlatlons obtained with different amounts of these compounds were In the range of 0.63 to 5.36% ( n = I O ) . The reactlon mechanism Is also discussed.
Carbodiimides are widely used as excellent coupling or dehydrating agents in synthesis (1-3) and analysis (3-10). Recently, Wilchek and co-workers (10,II) have demonstrated their cleaving activity on pyridine (eq 1) and established a colorimetric method for the determination of carbodiimides (IO). Since then the author, using the same reaction, has developed a fluorometric method for the determination of malonic acid (9). At present, the cleavage of pyridine to form glutaconaldehyde for colorimetry is carried out with cyanogen halide (12,13),pyridylpyridinium dichloride (14),or gem-polyhalogen compounds (15-18). However, few reagents have been reported for breaking the pyrimidine ring. In the present study the author, interested in the wide occurrence of nitrogen aromatic heterocycles and also in the cleaving activity on the pyridine ring as well as in the chromogenic reaction of the cleft product with dimethylbarbituric acid (DMBA), has investigated the action of dicyclohexylcarbodiimide (DCC) on some nitrogen aromatic heterocycles and developed a colorimetric
method for the determination of pyridyl and pyrimidinyl compounds. EXPERIMENTAL SECTION Apparatus. A Shimadzu UV-21OA double beam spectrophotometer and a Shimadzu RF-520 dual-beam difference spectrofluorometer equipped with a 150-W xenon lamp and 1-cm quartz cells were used for the determination of absorbance and fluorescence intensity, respectively. The wavelengths indicated were uncorrected. Mass spectrometry w a ~ performed on a Hitachi M-52 mass spectrometer. The electron ionization energy was 20 eV. The final step of the purification of the chromophore was carried out on a Hewlett-Packard 1084B liquid chromatograph equipped with a Hewlett-Packard 79850B LC terminal and a reversed-phase RP-18 column (5 bm, 250 X 4.6 mm; E. Merck) with methanol as the eluent. Materials and Reagents. Nitrogen aromatic heterocycles were purchased from Tokyo Kasei (Tokyo,Japan; 2-aminopyrimidine, pyrimidine, pyrazole, phthalazine, pyridazine, imidazole, 2aminopyridine,and isoquinoline),Wako Pure Chemicals (Osaka, Japan; piperidine and quinoline),Kanto Chemical (Tokyo,Japan; sulfadiazine, sulfamerazine, and sulfamethazine), E. Merck (Darmstadt, West Germany; pyridine and pyrrole), Aldrich (Milwaukee, WI; 2-amino-4-methylpyrimidine and 2-amino-6methylpyridine),Sigma (St Louis, MO; pyridoxine HCl),and local drug stores (pharmacopial grade; sulfaphenazole, sulfadimethoxine, sulfisoxazole,sulfaguanidine, sulfamoxole, sulfisomidine, sulfamethoxazole, sulfamethoxypyridazine, chlorpheniramine maleate, niacin, isoniazid, and thiamine HCl). All of these compounds were used as received. The reagents, DCC and DMBA, were obtained from Tokyo Kasei and Fluka (Buchs, Switzerland), respectively, and were dissolved in methanol (E. Merck) without further purification. Thin-layer chromatography for the identification of the chromophore from sulfadiazine and malonaldehyde bis(di-
0003-2700/85/0357-1461$01.50/00 1985 American Chemical Society
